Consumable magnesium anode with a tin-coated, ferrous metal core wire

ABSTRACT

A consumable magnesium anode is provided which is particularly suitable for use in a household water heater. In a preferred embodiment, the magnesium anode is extruded over a steel core wire coated with tin metal. Prior to extrusion, the core wire is heated at a temperature which will convert the tin metal coating to a tin-iron alloy coating having a melting point high enough to permit successful extrusion of the anode rod around the core wire. Altering the composition of the tin metal coating by the heating step does not destroy the galvanic compatibility of the tin coating with the magnesium.

United States Patent [1 1 George [111 3,742,588 (451 July 3, 1973CONSUMABLE MAGNESIUM ANODE WITI-I A TIN-COATED, FERROUS METAL CORE WIRE[75] Inventor:

[73] Assignee: The Dow Chemical Company,

' Midland, Mich.

221- Filed! May 6, 1971 [21] Appl. No.: 140,672

Percy F. George, Lake Jackson, Tex.

[52] U.S. Cl... 29/458, 29/474.4, 29/480 [51] Int. Cl 823p 3/10 [58]Field of Search 29/458, 527.3, 527.2, 29/196.4, 474.4, 480; 204/197;117/128, 114 B, 231

[56] l References Cited UNITED STATES PATENTS 2,735,l63 I 2/1956 Brookset al 29/527.3 X 3,099,083 7/l963 De Long....... 29/458 3,558,463

1 1971- -Strobach et al 174/84 x OTHER PUBLICATIONS I Constitution ofBinary Alloys, Hansen, 1958 pp. 718,

Primdry Examiner-Charles W. Lanham Assistant Examiner-D. C. CraneAttorney-Griswold & Burdick, V. Dean Clausen and Lloyd S. Jowanovitz 5 7ABSTRACT A consumable magnesium anode isprovided which is particularlysuitable for use in a household water heater. In a preferred embodiment,the magnesium anode is extruded over a steel core wire coated with tin-metal. Prior to extrusion, the core wire is heated at a temperaturewhich will convert the tin metal coating to a tin-iron alloy coatinghaving a melting point high.

enough to permit successful extrusion of the anode rod around the corewire. Altering the composition of the tin metal coating bythe heatingstep does not destroy the galvanic compatibilityof the tin coating withthe magnesium;

5 Claims, 1 Drawing Figure Ex fruc/eo ma flea/um CONSUMABLE MAGNESIUMANODE WITH A TIN-COATED, FERROUS METAL CORE WIRE BACKGROUND OF THEINVENTION This invention relates broadly to a consumable anode. Morespecifically, the invention covers a magnesium anode for a water heater,in which the anode is cast or extruded over a ferrous metal core wirecoated with tin metal.

Consumable magnesium anodes are widely used for cathodic protection ofiron or steel structures, such as underground pipelines, the hulls ofships and the inside wall surfaces of household water tanks. The anodestructure used in a water heater usually consists of a magnesium rodextruded over a steel core wire. In operating position, the anode rod isusually suspended from the upper tank wall by connection to a hot wateroutlet fitting, with the core wire also being connected into thefitting. The core wire thus provides the required galvanic connectionbetween the anode rod and the tank wall. Additionally, the core wireassures electrical connection to those parts of the anode which remainafter some of the anode material has been completely consumed.

On the inside of, the water heater tank substantially more bare metal isexposed at the top and bottom of the tank than is exposed along the sidewalls. The extra bare metal area is created by the top and bottom headsof the tank, by the various inlet and outlet fittings and by flaws inthe glass lining from welding of the top and bottom heads to the sidewalls. The additional current needed to protect the bare metal area fromcorrosion causes the ends of the anode to corrode first. Eventually, theends of the anode will corrode down to the bare core wire and additionalcurrent flow is created between the anode and the core wire itself. Theresulting accelerated corrosion of the anode shortens its lifeconsiderably.

Past attempts to construct an anode which is not subject to the problemsmentioned above have not been entirely satisfactory. One attempt is ananode invention described in U.S. Pat. No. 2,841,546, to Robinson, inwhich the steel core wire is coated with aluminum prior to extrusioninto the magnesium anode rod. An undesirable feature of thisconstruction is that during extrusion the aluminum coating on the corewire forms an eutectic compound with the magnesium anode rod, whichplugs the wire entrance hole into the extrusion die and causes the corewire to break. Another disadvantage is that the iron in the core wirewill diffuse into the aluminum coating to give a composition which hasvery poor galvanic compatibility with the magnesium anode. Earlierattempts, as noted in the Robinson patent, included applying a zincgalvanized coating to the steel core wire prior'to extrusion into themagnesium coating, which is molten during the extrusion process, reactswith the metal of the extrusion die and causes 1 the die to crack fromstress corrosion.

Although tin occupies a more noble location than magnesium intheelectromotive series, it has been structure which comprises a magnesiummetal article and a ferrous metal article. The suppression is achievedby interposing a continuous layer of tin metal between the magnesiummetal and the ferrous metal. The De- Long patent, however, does notteach the present in-' vention. The invention described hereincovers animproved method for extruding or casting a magnesium article over atin-coated ferrous metal by heating the ferrous metal article prior toextrusion or casting to alter the composition of the tin coating.

SUMMARY OF THE INVENTION A broad object of the invention is to providean improved method for fabricating a consumable magne sium anode havinga ferrous metal core wire. 1

A more specific object is to provide a method for fabricating amagnesium anode in which the ferrous metal core wire is coated with tinand heated prior to extruding or casting the anode rod around the corewire.

Broadly, the invention provides a consumable anode fabricated of aferrous metal core wire encased by' a magnesium metal sheath. Apreferred method for fabricating the anode comprises applying acontinuous coating of tin metal to the core wire. The tin-coated corewire is then heated in a nonoxidizing atmosphere at a temperature belowthe melting point of the tin. The heating period is for a timesufficient to convert the tin metal coating to a tin-iron alloy coatinghaving a melting point not lower than about 800 F. After'obtaining thedesired tin-iron alloy composition on the core wire, the magnesium metalsheath is shaped around the tincoated wire.

DESCRIPTION OF THE DRAWING The single FIGURE is a cross-section view ofa con- I sumable magnesium anode fabricated according to the anode. Theproblem in this concept isthat the zinc,

known for some time that when the two metals are galvanically coupledthe tin is suitably compatible with the magnesium. One reference whichdescribed this phemethod of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT The drawing illustrates across-section view of a consumable anode of this invention, as indicatedgenerally by numeral 10. Basically, the anode 10 consists of a ferrousmetal core wire 11, which has a continuous tin metal coating 12 thereon.The tin-coated core wire 11 is encased by a magnesium anode rod 13. Therod 13 may be shaped around the core wire by a conventional extrusionprocess or by casting. In practice, it is preferred to extrude the anoderod over the core wire.

In fabricating the anode structure, it is preferred to use commerciallyavailable anode core wire of the usual composition, such as black ironor steel. For the anode rod, the preferred magnesium alloy compositionis AZ3lB, as designated by the American Society for Testing Materials(ASTM). For the continuous tin metal coating applied to the core wire,the preferred composition is a commercial tin "alloy compositioncomprising at least 40 percent tin. The tin metal coating may be appliedto the core wire by any of various known commercial methods, such asspraying, hot dripping, casting or electroplating. A suitable thicknessfor the tin metal coating is from about 0.0002 to 0.0003

of an inch. It is especially preferred to coat the steel core wire withtin metal to a thickness of about 0.0002 I of an inch, using acommercial electroplating process.

Referring to extrusion of the anode rod'over the core enough to permit asuccessful extrusion without destroying the galvanic compatibility ofthe tin composition with the magnesium. Since tin melts at about 449 F.and the extrusion temperature of magnesium is about 800 F., a successfulextrusion can be obtained only by altering the composition of the tincoating. In practice, this is achieved by heating the tin-coated corewire for a given period of time at a temperature below the melting pointof tin, to thereby diffuse a small amount of iron into the tin coating.The resulting tiniron alloy coating has suitable galvanic compatibilitywith the magnesium anode rod and the melting point of the alloy is abovethe 800 F. required for extrusion. Specifically, the tin-coated corewire is heated in a nonoxidizing atmosphere at about 425 F. for fromabout 2 hours to 8 hours to obtain a tin-iron alloy coating having amelting point of between about 800 F. and l,000 F. For optimum extrusionconditions, it is preferred to heat the core wire for about 8 hours at425 F., to obtain a tin-iron alloy coating having a melting point ofabout 1,000 F.

Several tests were conducted to determine the galvanic compatibility andother characteristics of a consumable anode structure fabricatedaccording to the practice of this invention. The first test, which isdescribed in Example I below, relates to galvanic compatibility of atin-coated ferrous metal cathode with a magnesium anode.

EXAMPLE 1 Pieces of steel measuring about 1 inch wide, 3 inches long and0.007 of an inch thick, were used as a cathode. Three (3) differenttypes of'cathode pieces were used in the test. The first type was anuncoated steel piece, which was designated Cathode A. The second type,which was designated Cathode B, was a steel piece coated with tin metalabout 0.0002 of an inch thick. The Cathode B pieces were not heattreated prior to the galvanic compatibility test. The third type, whichwas designated Cathode C, was a steel piece having a tin coatingidentical to Cathode B. The Cathode C pieces were heated at 500 F.(i.e., above the melting point of tin) for from about 1 hour to 6 hoursprior to the galvanic compatibility tests.

Each cathode piece was galvanically connected by a metal fastener to apiece of magnesium (AZ3lB) of identical size, which had been preciselyweighed. The metal coupled unit thus formed a galvanic cell, with themagnesium piece representing the anode. Each cell was placed in a waterelectrolyte, so that about 1 inch of the cell projected above thesurface of the water. The cell was held in this position in theelectrolyte solution for a period of about 24 hours. The waterelectrolyte had an electrical resistivity of about 450 ohm/cc and wasmaintained at a temperature of 150 F. to simulate average conditionsfound in a household water heater. At the end of the 24-hour period,each cell was disassembled and each magnesium anode piece was thoroughlycleaned in a chromic acid solution. Each anode piece was again preciselyweighed to determine loss from galvanic corrosion during the period thecell was immersed in the water electrolyte. Results are shown in Table 1below.

TABLE I Galvanic Compatibility of Mg Anode-Steel Cathode Unit of ExampleI Net Wt Cathode Heat Treatment of Cathode Loss of Type Temperature TimeMg Anode A 131 g. B 0.52 g C 500F. 1 hr. 0.33 g C 500F. 3 hrs. 0.46 g C500F. 6 hrs. 0.74 g

From the data in Table I it will be apparent that the tin metal coatingon the steel cathode piece has a defi nite corrosion-inhibiting effecton the magnesium anode. It will be noted, for example, that in the cellunit containing the uncoated steel cathode (Cathode A), the weight lossof the magnesium anode was more than double that of the magnesium anodein the cell units containing tin-coated cathodes.

EXAMPLE 2 The following test was to determine the galvanic compatibilityof the magnesium anode with a tin-plated cathode in which thecomposition of the tin coating was altered by heating the cathodestructure at a temperature below the melting point of tin. The anode andcathode pieces employed in this example were identical in size to thosedescribed in Example 1 and the galvanic cell units were made upaccording to the procedure of Example 1. In addition, the compatibilitytest of each cell unit was conducted under the same conditions describedin Example 1. Prior to the compatibility test each of the tin-platedcathode pieces was heated at 425 F. for a periodof time sufficient toconvert the tin coating to a tin-iron alloy coating. The melting pointof each alloy coating was then checked by a conventional procedure todetermine if it was high enough for successful extrusion of the cathodewith a magnesium metal anode. Results are set out in Table ll below.

TABLE II Galvanic Compatibility of Mg Anode-Steel Cathode Unit ofExample 2 Melting Point. Net Wt Heat Treatment of Tin-Iron Loss of ofCathode Alloy Coating on Mg Cathode Type Temp. Time. Cathode AnodeTin-Plated Steel 425F. l hr. 600F. 0.26 g. Tin-Plated Steel 425F. 2 hrs.800F. 0.33 g. Tin-Plated Steel 425F. 8 hrs. 1000"F. 0.43 g.

EXAMPLE 3 The test of this example relates to current flow between themagnesium anode rod and the tin-coated steel core wire of an anodestructure fabricated according to this invention, as compared to currentflow between a magnesium anode and an uncoated steel core wire. Twotypes of anode structures were prepared by a conventional extrusionprocess. One of the anode structures, which was designated anode A,consisted of a magnesium anode extruded over a bare steel core wire,(i.e., the core wire was not coated with tin metal and it was notheat-treated). For this test the anode structure A served as a controlblank. The second anode structure, which was designated as anode B,conabout 9 days. A standard condition for the current flow measurementwas established by using a factor to correct the water resistivity to'anaverage of 450 ohms/cc and the water temperature to an average of 58 C.Th

sisted of a magnesium anode extruded over a steel core 5 results areShown in Table m below:

('urrent Flow From Mg Anode Rod to (ore Wire in the Anode Structure ofExample 3 Current Flow From Mg Anode to 'lank Wall +6 in. Exposed (ToreWire (in milliamperes) TABLE 111 Difference in ('urrent Required fromAnode (/\-Bl Current Anode Structure A Anode Structure B MeasurementPeriod (Uncoated Steel Core (Tin-Plated Steel Core Percent (in Days)Wire) WireHeat Treated) Milliamperes (approximate) 1st 27.0 24.0 3.011.0 2nd 31.0 24.3 6.7 21.5 5th 28.0 23.0 5.0 18.0 7th 25.0 20.0 5.020.0 8th 26.5 23.0 3.5 13.2 9th 27.0 22.0 5.0 18.5

wire, which had been plated with a tin metal coating of about 0.0002inch thickness. The core wire of anode B was heated in a nonoxidizingatmosphere at about 425 F. for about 8 hours prior to extrusion with themagnesium anode.

Each of the anode structures A and B were installed in a conventionalglass-lined water heater and were set up to test for current flowbetween the anode rod and the core wire in the following manner. Theanode structure was suspended from the upper tank wall of the heater byconnection of one end to a fitting which was insulated from the tankwall itself. The fitting was insulated to prevent contact between themagnesium anode and the steel tank wall of the heater. The lower six (6)inches of the magnesium anode rod was carefully stripped away from thecore wire. The exposed piece of core wire was cut off and thenreconnected at its cut end to the core wire remaining in the anode rod.1n the reconnection the cut end of the core wire was insulated from theanode rod and the main core wire remaining in the anode rod. Theopposite end of the exposed core wire piece was connected to one end ofa piece of heavily insulated copper wire. The opposite end of the copperwire was connected directly to the inner side wall of the heater tank.

The upper end of the main core wire in the magnesium anode was extendedbeyond the anode and was connected through a milliammeter to the uppertank wall of the heater. The milliammeter thus enabled measurement ofthe current flow from the anode rod to the tank wall, which alsoincludes any stray current flow from the anode rod to the exposed corewire piece.

From the data of Table 111 it will be observed that the exposed corewire of anode structure B, which was tincoated and heat-treatedaccording to the practice of the invention, demanded substantially lesscurrent from the anode than the exposed uncoated core wire of anodestructure A. As indicated, in a comparison between the two anodestructures, the average difference in current flow from anode to tankwall plus core wire is more than 15 percent. The substantially lesseramount of current demanded by the tin-coated core wire, therefore,contributes greatly to the life of the anode.

What is claimed is:

1. In the fabrication of a consumable anode consisting of a ferrousmetal core wire-encased by a magnesium metal sheath, the method whichcomprises:

a. applying to the ferrous metal core wire a continuous coating of a tinmetal,

b. heating the tin-coated core wire in a non-oxidizing atmosphere at atemperature below the melting point of tin for a period of timesufficient to convert the said tin metal coating to a tin-iron alloycoating having a melting point not lower than about 800 F., and

c. extruding the magnesium metal sheath around the said tin-coated corewire.

2. The method of claim 1 in which the core wire is a steel wire.

3. The method of claim 1 in which the tin-metal coat ing on the corewire has a thickness of from about 0.0002 of an inch to 0.0003 of aninch.

4. The method of claim 1 in which the tin-coated core wire is heated atabout 425 F. for from about 2 hours to 8 hours.

5. The method of claim 1 in which the tin-iron alloy coating has amelting point of about 1,000 F.

2. The method of claim 1 in which the core wire is a steel wire.
 3. Themethod of claim 1 in which the tin-metal coating on the core wire has athickness of from about 0.0002 of an inch to 0.0003 of an inch.
 4. Themethod of claim 1 in which the tin-coated core wire is heated at about425* F. for from about 2 hours to 8 hours.
 5. The method of claim 1 inwhich the tin-iron alloy coating has a melting point of about 1,000* F.